A valve train for an internal combustion engine which can change the valve operating properties of engine valves is disclosed in, for example, Japanese Patent Unexamined Publication No. JP-A-58-214610. The valve train so disclosed includes a rocker arm (hereinafter, referred to as a primary rocker arm) supported in an oscillatory fashion on a fixed point or fulcrum which is eccentric to a rocker shaft and adapted to be oscillated by a primary cam which rotates in synchronism with the rotation of the engine and a oscillating cam which is rotatably supported on a camshaft which is in parallel with the rocker shaft. A cam profile made up of a base circle portion where an inlet valve remains not lifted and a lifting lobe portion where the inlet valve is lifted and a contact surface with which the primary rocker arm is brought into abutment are formed on the oscillating cam which opens and closes an inlet valve provided in a cylinder head. The inlet valve is opened and closed in accordance with rotational positions of the primary cam when the valve drive force of the primary cam is transmitted to the oscillating cam via the primary rocker arm. Then, opening and closing timings and maximum lift amount of the inlet valve are changed by displacing the fulcrum. Here, it is understood that the camshaft, which supports the oscillating cam, is not displaced relative to the cylinder head.
For other conventional apparatuses for changing the valve operating properties of engine valves of internal combustion engines, there are apparatuses which are disclosed, for example, in Japanese Patent Unexamined Publications Nos. JP-A-7-91217, and JP-A-5-71321. An apparatus disclosed in the JP-A-7-91217 includes a drive shaft which is driven to rotate by an internal combustion engine, a camshaft which is provided on an outer circumference of the drive shaft in such a manner as to rotate freely relative to the drive shaft and which has a cam for actuating an inlet valve to be opened and closed, a disk housing provided so as to oscillate freely about a pivot support pin as a fulcrum in a radial direction relative to the drive shaft, an annular disk rotatably supported on an inner circumferential surface of the disk housing, a drive mechanism for oscillating the disk housing and a rocker arm which is pivot supported in an oscillatory fashion on a rocker shaft which is supported on the disk housing at one end portion thereof and which abuts with the cam and the inlet valve. Then, when the disk housing is cause to oscillate by the drive mechanism, the center of the annular disk becomes eccentric to the axial center of the drive shaft, whereby the angular velocity of the camshaft is changed, and then the operation angle of the inlet valve is changed. At the same time, due to the displacement of the rocker shaft which oscillates together with the disk housing, the pivot support point of the rocker arm is changed, and the other end portion of the rocker arm shifts in a diametrical direction on an upper surface of a valve lifter, whereby a rocker ratio relative to the inlet valve is changed, the valve lift amount being thereby changed.
In addition, a variable valve train disclosed in the JP-A-5-71321 includes a rocker arm which is brought into contact with a rotating cam and an inlet valve, a lever which is rotatably supported on a fulcrum shaft so as to be joined to a back side of the rocker arm in an oscillatory fashion, a link which connects the fulcrum shaft to the rocker arm and a controller cam which changes over the position of the lever from a high lift position where the position of the lever approaches the cam to a low lift position where the position of the lever moves apart from the cam. In a state where the rocker arm contacts a base circle of the cam, a distal end of a joint portion of the lever which connects a point where the lever contacts the rocker arm at a low lift position to a point where the lever contacts the rocker arm at a high lift position is formed into a concentric arc-like sectional shape which is formed about the fulcrum shaft, and a joint portion of the rocker arm which contacts the inlet valve is formed into a concentric arc-like sectional shape. Then, by changing over the lever position to the low lift position or high lift position, the valve lift amount of the inlet valve is changed.
In valve trains of internal combustion engines, a clearance is provided, for example, between an engine valve and a rocker arm which abuts with the engine valve or between a cam and a rocker arm which abuts with the cam and an engine valve.
In the conventional valve train that has been described in the JP-A-58-214610, the cam profile of the oscillating cam abuts with a valve lifter, which is a member on the inlet valve side. This is because the cam profile of the oscillating cam cannot be brought into abutment with the inlet valve as the shift amount of an abutment position where the cam profile abuts with the member becomes large between the cam profile and the member which abuts with the cam profile, when the operating angle and lift amount (valve operating properties) of the inlet valve are changed. Thus, in the conventional valve train, since the cylindrical valve lifter with which the oscillating cam is brought into abutment and a holding portion for holding the valve lifter slidably need to be provided in the cylinder head, the cylinder head is enlarged. Due to this, in an internal combustion engine in which the width of the cylinder head is narrow in a direction which intersects at right angles with a plane which includes cylinder axes of the internal combustion engine and which is in parallel with the rotational center line of the primary cam, it is difficult to install such a valve train while maintaining the compactness of the internal combustion engine.
In addition, a consideration is given to a valve train in which a separate rocker arm is adopted in place of the oscillating cam in the aforesaid conventional valve train for abutment with the inlet valve, and the separate rocker arm is made to be oscillated by the primary rocker arm. In this case, since the necessity of the valve lifter is obviated, it becomes possible for the valve train to be applied to the internal combustion engine which is narrow in the direction which intersects at right angles with the plane. However, since the fulcrum of the separate rocker arm is not displaced in contrast to the primary rocker arm whose fulcrum is displaced, it becomes difficult to maintain a clearance between the abutment portion of the primary rocker arm and the abutment portion of the separate rocker arm or the abutment state therebetween when the valve operating properties of the inlet valve are changed, thereby making it difficult to maintain an appropriate valve clearance. As a result, for example, due to an increase in valve clearance, noise is increased due to striking noise generated when the inlet valve starts to be opened, and noise is also increased due to collision of the rocker arms with each other when the internal combustion engine vibrates In addition, irrespective of a change in the valve operating properties, when attempting to maintain the clearance between the abutment portions or abutment state therebetween, the configurations of the abutment portions become complicated, leading to an increase in costs.
Furthermore, in the event that the fulcrum of the separate rocker arm is not displaced, the control range of valve operating properties is determined solely by the displace amount and displacement direction of the fulcrum of the primary rocker arm, and therefore, for example, when attempting to expand the control range of the opening and closing timings of the inlet valve, since the displacement amount of the primary rocker arm needs to be increased, the aforesaid maintenance of the appropriate valve clearance becomes more difficult, and therefore, the control range of valve operating properties cannot be actually set large.
Then, in the technique disclosed in the JP-A-7-91217, since the rocker arm abuts with the cam and the valve lifter, when the disk housing is caused to oscillate so that the rocker shaft oscillates together with the disk housing in order to change the operating angle and the valve lift amount (valve operating properties), while an abutment state is maintained between the rocker arm and the valve lifter, the clearance between the cam and the rocker are changes, and as a result, the valve clearance changes. In addition, in the technique disclosed in the JP-A-5-71321, since the rocker arm abuts with the cam and the inlet valve, when the position of the lever is changed over so that the rocker arm pivot supported by the link rotates about the fulcrum shaft in order to change the valve lift amount (valve operating properties), while the clearance or the abutment state is maintained between the joint portion of the rocker arm and the inlet valve, the clearance between the rocker arm and the cam changes, and as a result, the valve clearance changes.
Thus, in the valve train in which when the valve operating properties are changed, the oscillating center line of the rocker arm which abuts with the engine valve changes, when the valve operating properties are changed, the valve clearance changes. In this case, even in case the valve clearance is an appropriate value for a specific valve operating property, the valve clearance does not become an appropriate value in another valve operating property. Then, for example, when the valve clearance becomes larger than the appropriate value, noise is increased which results from striking noise generated when inlet and exhaust valves start to be opened.